Position measuring device for detecting displacements with at least three degrees of freedom

ABSTRACT

The invention comprises a fixed platform ( 1 ) and a displaceable platform ( 2 ) that are coupled by six tension springs ( 3 ) and an elastic spacing element ( 6 ), which forms with each platform, for instance, a ball-and-socket joint, so that the platforms can be displaced in a total of five to six degrees of freedom with respect to each other. Displacement is detected by measuring at the tension springs ( 3 ) or at the spacing element ( 6 ). This is preferably done by measuring the inductivity of the tension springs ( 3 ), thereby making it possible to easily determine the relative position of the platforms.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Swiss patent application2983/96, filed Dec. 12, 1996, the disclosure of which is incorporatedherein by reference in its entirety.

1. Technical Field

The invention relates to a position-measuring device.

Devices of this type are especially used as input or operatingapparatus, e.g. for operating screen graphics (e.g. for CAD systems) andcomputer animations, for controlling robots, for moving parts of tooland measurement machines (spindle boxes and measuring heads), as sensorsor for controlling remote controlled probes and surgical instruments.

2. State of the Art

In conventional devices, where displacements with three or even five tosix degrees of freedom are measured, complicated measuring electronicsare required, which makes the devices more expensive and unwieldy, orsimpler measuring electronics are used, which, however, lead tounsatisfactory ergonomic properties. Examples of such devices are givenin U.S. Pat. No. 4,811,608, EP 244 497, EP 240 023 and EP 235 779. Inall these devices, optical, mechanical or electrical sensors arerequired, which must additionally be housed in the device and lead to acorrespondingly complicated setup.

SUMMARY OF THE INVENTION

Hence, it is an object of the invention to provide a device of the typementioned above that avoids these disadvantages.

Hence, parameters of the elastic coupler are measured directly, such asforces, electrical properties, etc. In this way, separate sensors can bedispensed with or be designed in very compact manner, since the couplingdevice itself forms at least a part of the sensors.

In a preferred embodiment several inductivities of the coupler, or ofparts of the coupler are measured. Thus, for instance, the inductivityof springs of the coupler depending on the dilatation is measured.

Further electric parameters that can be measured are the electricresistance or the capacity of parts of the coupler.

Since three or more parameters must be measured for detecting theposition or orientation with three or more degrees of freedom, theseparameters are preferably measured sequentially, such that theindividual measurements cause no mutual interferences and the apparatusremains simple.

The coupling device preferably comprises several spring members, inparticular springs, which movably hold the two reference members at adistance from each other with the desired number of degrees of freedom.In a simple and therefore preferred embodiment, several extensionsprings and a spacer member are e.g. provided. The spacer member isconnected in articulated manner to one or both reference members, e.g.via ball-and-socket joints. Depending on the number of the desireddegrees of freedom, the spacer member can be compressible along itslength.

The device is preferably designed such that the possible mutualdisplacement of the reference members upon an actuation by hand isperceived to be comparatively large, i.e. that it is as least 1centimeter or 20° in each degree of freedom. Such displacements aredistinctly perceived by a human user and allow a secure operation of thedevice.

The device according to the invention is especially suited as an inputdevice for computers, a control device or a measuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and applications of the invention result from the nowfollowing description making reference to the annexed drawings, wherein:

FIG. 1 is a first embodiment of the invention,

FIG. 2 is a detailed view of the spacer member of the embodiment of FIG.1,

FIG. 3 is a block diagram of a circuit for measuring the springinductivity,

FIG. 4 is a spring with metal core,

FIG. 5 is a spring with metal shell,

FIG. 6 is a spring with a capacitive measuring arrangement,

FIG. 7 is a spring with force sensor,

FIG. 8 is a second embodiment of the invention,

FIG. 9 is a side view onto a capacitive measuring arrangement for theembodiment of FIG. 8,

FIG. 10 is a top view onto the device of FIG. 9,

FIG. 11 is a third embodiment with only five degrees of freedom,

FIG. 12 is a fourth embodiment of the invention,

FIG. 13 is a fifth embodiment of the invention with extension springs,

FIG. 14 is a sixth embodiment of the invention with pressure springs,

FIG. 15 is a further embodiment of the invention with a total of ninesprings,

FIG. 16 is an alternative to the embodiment of FIG. 15 with coveringbellow from above, wherein only the right half of the figure is shown,

FIG. 17 is a vertical section along line XVII—XVII of FIG. 16, and

FIG. 18 is the attachment of the springs of the embodiment of FIG. 15.

METHODS FOR CARRYING OUT THE INVENTION

A first embodiment of the device according to the invention is shown inFIG. 1. Here, only those parts are shown that are of significance forthe suspension and the actual measurement. Provided with a handle thedevice can e.g. be used as computer mouse with up to six degrees offreedom, i.e. as a hand sized apparatus, the displacements of which aregenerated by one hand and are measured and transferred to a targetsystem. Further applications are listed at the end of the description.

The device comprises two platforms 1, 2, which act as the referencemembers, the mutual position of which is determined. Platform 1 is inthe following called the fixed platform, platform 2 the movableplatform. However, platform 2 could also be fixed and platform 1movable, or both platforms can be arranged in movable manner.

Six schematically shown extension springs 3 are arranged between the twoplatforms, preferably coil springs made of steel or copper alloys. Theextension springs 3 are not parallel to each other, nor are theyparallel to a single plane. The extend from three lower points 4 offixed platform 1 to three upper points 5 of movable platform 2. Theylower and upper points are preferably approximately on the corners of aequilateral triangle, wherein the triangle of the lower points 4 isrotated about 60° in respect to the one of the upper points 5. Twoextension springs 3 extend from each lower point 4, one to each of theneighboring upper points 4. It is also possible to arrange the extensionsprings in another manner between the platforms, wherein they are, inthis embodiment, preferably not parallel and chosen such that therelative position of the two platforms can be calculated from theirlengths.

A spacer member 6, as shown in FIG. 2, is located between platforms 1, 2and in the center of the extension springs 3. It comprises a lower ball7 and an upper ball 8, which lie in corresponding holes 9 and 10 of theplatforms 1, 2 and form two ball-and-socket joints with the same. Lowerball 7 is rigidly connected to a rod 11, to which upper ball 8 ismounted in axially displaceable manner. A (schematically shown) pressurespring 12 designed as a coil spring extends between balls 7 and 8. Inmounted state as shown in FIG. 1, pressure spring 12 is biased and urgesupper ball 8 and therefore upper platform 2 upwards. Hence, pressurespring 12 acts against the force of the extension springs 3.

In the embodiment of FIG. 1, upper platform 2 can be moved in respect tolower platform 1 in all three translational and all three rotativedegrees of freedom because the spring elastic coupler consisting ofspacer member 6 and the extension springs 3 allow displacements in allrotative and translational directions.

In an application as input device for computers, the lower, fixedplatform 1 can rest on a table, while the user actuates a handlearranged on the upper, movable platform 2. The displacements (i.e. therotations as well as the translations) of the movable platform 2 can bedetected by differing methods as explained in the following.

In a preferred embodiment of the invention, the displacement or motionof the upper platform is calculated by measuring the inductivity of thetension springs 3. For this purpose, the relation is used that theinductivity L_(F) of a coil shaped spring is approximately proportionalto z·W/g, wherein z is the number of windings, W the winding surface andg the distance between windings. The inductivity L_(F) is thereforeapproximately proportional to the reciprocal length 1 _(F) (cf. FIG. 1)of the spring body. Therefore, by measuring the inductivity of alltension springs 3, their lengths 1 _(F) can be determined. From thesesix lengths 1 _(F) and from the stored configuration information of thedevice (i.e. the sizes of the two triangles formed by the lower points 4and the upper points 5 or the relative positions of the springsuspension points on the corresponding platforms) the relative positionof the two platforms 1, 2 can then be calculated.

FIG. 3 shows a circuit for determining the inductivity of the tensionsprings 3. Here, each tension spring 3 forms the inductivity L_(F) of aLC-oscillator 20. For this purpose, the ends of the springs areconnected with feed wires, which are not shown in FIG. 1.

The frequency of each LC-oscillator 20 is given in known manner by theinductivity L_(F) and its parallel capacity. From the frequency and thegiven value of the capacity, the value of the inductivity L_(F) cantherefore be calculated.

Each oscillator 20 possesses a control input, by means of which it canbe switched on and off. In switched off state, the oscillator is notoscillating and its output is on high impedance. When the oscillator isswitched on, it is oscillating and generates an output signal. Theoutputs of the oscillators 20 are connected to each other and are led toa frequency counter 22.

In operation, control 21 operates the oscillators 20 in sequentialphases of measurement one after the other. In each phase, only oneoscillator 20 is in operation and its frequency is measured by frequencycounter 22 and then fed to a computer (not shown). In this way, theinductivities L of all tension springs 3 can be determined one after theother in six measuring phases. This sequential operation avoids that themeasurements of the individual springs interfere with each other.Furthermore, only a single frequency counter 22 is required.

In the present embodiment, springs with a diameter of 5 mm, a number ofwindings and, depending on extension, a distance between windingsbetween approximately 0.5 and 1.0 mm are used, i.e. the inductivityL_(F) is in the order of some μH. The oscillators are dimensioned suchthat their frequencies are in the range of several megahertz. In thisway, an accurate measurement or frequency count can e.g. be carried outwithin a millisecond.

In order to make the effect of the change of inductivity of the springsstronger, each tension spring 3 can be provided with a core 30 or shell31 of high magnetic permeability, as it is shown in FIGS. 4 and 5. Thecore 30 or shell 31 can e.g. be attached at one end to a coil of thespring, such that it maintains its vertical position.

Instead of the inductivity, other electric parameter of the coupler 3, 6can be measured as well. Since the specific electric resistance ofspring steel increases upon deformation, the lengths 1 _(F) of thetension springs 3 (and/or the pressure spring 12) can e.g. also bedetermined from their electric resistance R_(F). Also this measurementis again carried out sequentially such that the complexity of thecircuit is reduced.

Finally, electric capacities of the coupler 3, 6 could be measured aswell. In this case, an arrangement according to FIGS. 4 or 5 could beused, too, wherein the core 30 or the shell 31 are insulated againstspring 3 and are used as one electrode of a capacitor. The secondelectrode of the capacitor is then formed by the spring. The capacityC_(F) of the capacitor formed in this way depends on how many of thewindings are located in the area of core 30 or shell 31, respectively.The capacity measurement is again preferably carried out in sequentialmanner.

A further arrangement for a capacitive measurement is shown in FIG. 6.Here, the spring 3 is surrounded by two shells 31 a, 31 b, which areinserted telescopically into each other and electrically insulated fromeach other. One shell 31 a is attached to the upper and the other shell31 b to the lower end of the spring. The capacity of the capacitorformed by the two shells 31 a, b depends in linear manner from thelength of the spring. The telescopic arrangement of FIG. 6 does notnecessarily have to be arranged around a spring.

In the embodiment of FIG. 6, the spring 3 can also be dispensed with. Inthis case, the shells 31 a, 31 b are connected to the platforms 1, 2 andare in frictional contact with each other. A device with a coupler ofthis kind is not self-restoring, i.e. when platform 2 is moved and thenreleased, it will remain in its moved position.

Non-electric properties of the coupler 3, 6 can be measured as well inorder to determine its state of deformation. In particular, forces inthe coupler can e.g. be measured for this purpose. The extension springs3 can e.g. be provided with a force sensor 32, such as it is shown inFIG. 7. This sensor generates a signal that is proportional to thepulling force F_(F) of spring 3, from which the length of the spring canbe determined as well. A further example for such a device with forcemeasurement is described further below.

A mechanical Eigenfrequency or resonance frequency f_(F) of one or moreof the springs 3 can be determined as well. Since the Eigenfrequenciesof the springs depend on their state of extension, the length of thespring can also be determined by means of such a measurement.

The above methods of measurement can, of course, also be combined.Furthermore, measurements can also be carried out in the area of thespacer member 6 and, in particular, on its spring 12.

In the following, some further, preferred embodiments of the deviceaccording to the invention are discussed.

FIG. 8 shows an embodiment of the device with only three tension springs3 and a spacer member 6. The spacer member 6 is again located in thecenter of forces of the tension springs 3 and acts against their pullingforce.

The tension springs 3 are attached at their lower ends on three tongues35. Flexion and torsion sensors 36 are arranged on the tongues. Thetongues 35 are of a spring steel that is comparatively hard compared tothe springs and are only slightly deformed by the pulling forces of thesprings. The sensors 36 are designed such that they can not onlydetermine the absolute value but also the direction of the individualforce F_(F). From this quantity, the length and direction of thecorresponding tension spring and therefrom the position of the movableplatform 2 can be calculated. Preferably, three values are measured,from which the exact direction and magnitude of the pulling force F_(F)can be calculated completely. It is, however, also possible to carry oute.g. two measurements only, such that only two components or degrees offreedom of the pulling force are determined for each spring.

FIGS. 9 and 10 show an alternative, capacitive measurement of the stateof the springs of the embodiment of FIG. 8. Here, the tongues arearranged close above a printed circuit 50. Two or three measurementelectrodes 51 are arranged on printed circuit 50 below each tongue 35,the capacity of which in respect to the corresponding tongue 35 isdetermined. For achieving a measurement that is as linear as possible,an insulating ring 52 and an annular auxiliary electrode 53 are arrangedaround each measuring electrode 51, wherein the potential of theauxiliary electrode tracks the one of the measuring electrode such thatthe field of the measuring electrode becomes as homogeneous as possible.By measuring the capacity of two measuring electrodes 51 in respect toeach tongue 35, the torsion and flexion of the same can be determined.By using a third measuring electrode in position 54, the derivative ofthe flexion and thereby the end point of the spring can also bedetermined. It is also possible to measure the torsion only on the fixedplatform 1 and to measure the flexion on the movable platform 2. This isdone preferably without part 55, which generates a torque, i.e. thespring 3 is attached directly to tongue 35, such that the individualcomponents of spring 3 can be measured immediately.

Therefore, in the device of FIG. 8, several complementary values aremeasured, such that the total number of springs can also be smaller thansix, while still all the translational and rotative coordinates of themovable platform can be determined.

In the embodiments of the invention described so far, movable platform 2has a total of six degrees of freedom. This number can, however, also bereduced.

Thus, FIG. 11 shows a device with only five degrees of freedom. This isachieved by using a spacer member 6 a with constant length. Just as thevariable spacer member of FIG. 2, it comprises two balls 7, 8, both ofwhich are, however, rigidly connected to rod 11. Hence, the allowedsurface of displacement of movable platform 2 is restricted to thecalotte of a sphere.

In FIG. 12, a further embodiment is shown, where upper platform 2 hasonly three degrees of freedom in respect to lower platform 1. This isachieved by rigidly connecting spacer member 6 c with lower platform 1,while it forms a ball-and-socket joint 8 with upper platform 2 only.

As indicated in FIG. 12, this device can also be provided with a furtherlevel. For this purpose, platform 1 is e.g. placed on a socket 38, intowhich a conventional computer mouse displaceable in two dimensions isintegrated. Socket 38 rests on the surface of a table 39. Hence, thesurface of the table 39 can be considered to be a third reference memberof the device, in respect to which the second reference member can bedisplaced in two dimensions. Coupling between the first and thirdreference member can also be implemented in an other manner, such thate.g. displacements in three translational degrees of freedom arepossible as well.

FIG. 13 schematically shows an embodiment of the invention that usestension springs only. Here, fixed platform 1 is e.g. designed as a cupwith a bottom 41 and a cylindrical side wall 42, in which movableplatform 2 is suspended on a total of nine tension springs 43. Twotension springs 43 extend from each corner of the movable platform tothe upper rim of side wall 42 and one to floor 41. Also in thisarrangement, the lengths springs can e.g. be measured with the meansmentioned above. The application of nine springs has the advantage thateven large displacements still can be calculated robustly by means ofbalancing calculations.

FIG. 14 schematically shows an embodiment of the invention where onlypressure springs 12 a are used for connecting fixed platform 1 withmovable platform 2. Here, too, the deformation of the springs can bedetermined with the methods mentioned above, such that the displacementsof the joy stick type handle can be determined in two or three degreesof freedom. Preferably, for this purpose, the degrees of freedom of thehandle are limited to two or three, respectively.

FIG. 15 shows a further embodiment of the invention. In this embodiment,platform 2 is designed to be a hollow half sphere and can be used as ahandle. The coupler between platform 1 and platform 2 comprises ninecoil springs 60, 61. Six coil springs 60 arranged horizontally are usedas measuring elements by determining their inductivity in the mannerdescribed above. Each of the horizontal springs 60 is connected at oneend with a pin 62, which is rigidly anchored in platform 1. On its otherend, it is connected via a flexible connection member, i.e. a string ora wire 63, with platform 2. Each spring or wire 63 is deviated by a hook64 mounted to platform 1, such that the springs 60 can extendhorizontally while the strings or wires 63 are deviated from the planeof the springs 60. In this way, more room is available for the springs60. In addition to this, it is possible to house the springs in ahousing (not shown), for suppressing interfering signals.

Between platform 1 and 2 the strings or wires 63 extend in the samegeometry as the springs 3 of the embodiment of FIG. 1, such that therelative position of the two platforms 1, 2 can be calculated from thevariations of lengths in simple and numerically stable manner.

It is also possible to anchor the springs 60 at one end in the points 64and at their other end in platform 2 such that they take the place ofthe strings or wires 63. The strings or wires can also be dispensed withand hooks for deviation are not necessary anymore.

The coupler of FIG. 15 further comprises three vertical springs 61. Theyare anchored at one end in platform 1. At their other end, they are eachconnected via a wire or string 66 to platform 2. The wires or strings 66are deviated by three hooks 67. The hooks are located at the corners ofa triangular plate 68, which is resting on a column 69. Column 69 isrigidly connected to platform 1. The purpose of the parts 61, 66-69 liesprimarily in receiving the weight of platform 2 and in acting againstthe pulling force of the springs 60, i.e. they serve as a spacer memberbetween both platforms.

Depending on the frictional losses in the hooks 64 and 67, thearrangement of FIG. 15 can be self-restoring or not. If no automaticrestoration is desired, frictional losses are chosen to be large. If thefrictional losses are small, platform 2 automatically goes back into itsequilibrium position after a displacement.

The deviation for the springs 61 or their wires or strings 66 can bedispensed with as well if the springs extend directly between the points67 and the lower rim of platform 2.

Six vertical rods 71 are arranged along the periphery of platform 1. Atthe upper end of each rod 71, a safety string 72 is attached, which isconnected to platform 2. Rods 71 and strings 72 limit the range ofdisplacements of platform 2 in respect to platform 1.

It is also possible to provide e.g. a cylindrical wall instead of therods 71, extending along the periphery of platform 1. The strings 71then extend from the upper rim of the cylindrical wall to the lower rimof platform 2. In place of individual strings, a bellow can be used aswell, such as it is illustrated in FIGS. 16 and 17. Here, 80 designatesthe cylindrical wall, to the upper rim of which the bellow 81 isattached. Bellow 81 seals the device on its upper side. It consists ofan annular, foil-like, flexible material, which is dimensioned such thatit hangs loosely if the platform 2 is in its center position.Furthermore, radial ribs 82 are formed in the bellow 81, which are moretension proof than the remaining bellow. They take the place of thestrings 72 and limit the range of displacement of platform 2. The ribs82 can be worked into the bellow or e.g. extend below the bellow.

FIG. 18 shows a vertical cross section through a spring 60, as it e.g.is used in the embodiment of FIG. 15. For simplifying the set-up,platform 1 is designed as a printed circuit, onto which the measuringelectronics are placed. The springs 60 are made of material that can besoldered, preferably beryllium bronze. At their outer ends they end in astraight wire section 85. This wire section 85 is led through a hole inone of the pins 62 and from there to a soldering point 86 on platform 1.Behind pin 62, wire section 85 is bent such that the axial pulling forceof spring 60 is received by pin 62, i.e. pin 62 is used as an anchor. Inthis way, soldering point 86, which is connected to the evaluatingcircuitry, remains force free. Corresponding anchors of the springs canalso be used for the other embodiments of the invention shown here, atone end or a both ends.

In general, all the principles of measurement discussed here can also beused for input devices or joy sticks with only two or three degrees offreedom, respectively.

As mentioned initially, the device according to the invention can beused as an input element for computers of the type of a computer mouse.Another application of the device relates to a measuring sensor, thedisplacements of which caused by contact with an object to be measuredprovide complete information about the position and orientation of thesurface element that has been touched.

If the device is used as a computer mouse, two buttons in addition tothe known ones are preferably provided. These additional buttons can beused for switching the mouse on and off, such that the object moved bythe mouse does not fall back into its central position after releasingthe mouse.

The device can also be used as a measuring system for the continuoustracking of a robot, wherein one platform is mounted to the fixed andthe other to the moved part (e.g. a gripper hand) of the robot.

A further application relates to the control of vehicles, wherein thevehicle driver can control all possible displacements of the vehiclewith the device according to the invention instead of using theconventional separate control devices (steering wheel, gas and brakepedals, stick etc.).

The device can also be used for controlling cranes and robots.

The displacement of the movable platform can also be caused by otherparts of the human body but a hand, such as with one or both feet.

In the present embodiments spring members of metal, in particular a wellconducting material that can be soldered are used, such as berylliumbronze. It is, however, possible to use elastic elements of anothermaterial, in particular plastic.

While in the present application preferred embodiments of the inventionare shown, it is to be distinctly understood that the invention is notlimited thereto but can also be carried out in other manner within thescope of the following claims.

What is claimed is:
 1. Position measurement device comprising a firstand a second reference member, a plurality of springs extending betweensaid first and second reference members in such a way that said secondreference member is supported with respect to said first referencemember entirely by said springs and is displaceable with respect to saidfirst reference member with six degrees of freedom such that saidsprings change in length when said second reference member is displacedwith respect to said first reference member, and measuring meansconnected to said springs, said measuring means being adapted todetermine a relative position of said reference members in at leastthree degrees of freedom by measuring inductivities of said springs. 2.Position measurement device of claim 1, wherein said springs comprisecores disposed therein, said cores having a magnetic permeability suchthat for a change in length of said springs, a change of inductivity ofsaid springs is greater with said cores than without.
 3. Positionmeasurement device of claim 1, wherein said measuring means comprises atleast one LC oscillator circuit adapted to oscillate at a frequencydependent on said inductivities, said measuring means further comprisinga frequency counter adapted to measure said frequency.
 4. Positionmeasurement device of claim 1, wherein said measuring means comprises acontrol means adapted to measure said inductivities sequentially. 5.Position measurement device of claim 1, comprising at least six mutuallynon-parallel springs.
 6. Position measurement device of claim 1, whereinsaid second reference member comprises a handle adapted to bedisplaceable in translational degrees of freedom by at least 1 cm and inrotative degrees of freedom by at least 20 degrees.
 7. Positionmeasurement device of claim 1, wherein said springs comprise extensionsprings having first and second ends, and wherein said device comprisesattachment means adapted to attach said first and second ends of saidsprings to said first and second reference members, said attachmentmeans comprising anchor means adapted to receive a force of saidsprings, each of said springs further comprising wires connected to saidanchor means, said wires being further connected to one of said firstand second reference members such that said wires do not receive saidforce of said springs.
 8. Position measurement device of claim 1,comprising at least six springs between said first and second referencemembers.
 9. Position measurement device comprising a first and a secondreference member, a plurality of extension springs extending betweensaid first and second reference members in such a way that said secondreference member is supported with respect to said first referencemember entirely by said extension springs and is displaceable withrespect to said first reference member with six degrees of freedom suchthat said extension springs change in length when said second referencemember is displaced with respect to said first reference member, andmeasuring means connected to said extension springs, said measuringmeans being adapted to determine a relative position of said referencemembers in at least three degrees of freedom by measuring inductivitiesof said extension springs.
 10. Position measurement device of claim 1,wherein said springs comprise shells disposed about said springs, saidshells having a magnetic permeability such that for a change in lengthof said springs, a change of inductivity of said springs is greater withsaid shell than without.
 11. Position measurement device of claim 1,wherein said springs are coil springs.
 12. Position measurement deviceof claim 1, wherein said springs are mutually non-parallel.
 13. Positionmeasurement device of claim 1, wherein for each degree of freedom ofdisplacement of said second reference member with respect to said firstreference member, said measuring means is connected to one spring. 14.Position measurement device comprising a first and a second referencemember, a plurality of springs extending between said first and saidsecond reference members in such a way that said second reference memberis supported with respect to said first reference member entirely bysaid springs, is suspended in an equilibrium position of said springs,and is displaceable with respect to said first reference member with sixdegrees of freedom such that said springs change in length when saidsecond reference member is displaced from said equilibrium position, andmeasuring means connected to said springs, said measuring means beingadapted to determine a relative position of said reference members in atleast three degrees of freedom by measuring inductivities of saidsprings.
 15. Position measurement device of claim 1, wherein saidsprings are arranged such that a determination of a relative position ofsaid second reference member with respect to said first reference memberfrom measurements of lengths of said springs is enabled.